Comment by supertrope

Not much. People plan for the lowest common denominator. If service levels surge during certain days or hours that's great, but people don't want to get burned by a period of thin service. Imagine if Fedex said overnight mail could no longer be delivered on Thursdays in certain towns. You'd have to remember to check if today's Wednesday and if the recipient's town is a low service zone. Or you could just switch to a more consistently available network like UPS.